A rotating anode X-ray tube generates X-radiation in a diagnostic system, wherein the anode of the X-ray tube heats up upon operation and cools during exposure and afterwards.
The thermal heat flow and thermal cycling causes thermo mechanical distortion of the tube components. Therefore, the tube components have to be designed such that reliable operation is guaranteed under all specified conditions.
Many modern high performance X-ray tubes use hydrodynamic bearings to support the rotating anode and to dissipate the heat from the anode by direct conduction cooling towards an external cooling fluid. The loading capacity of these hydrodynamic bearings is a strong function of the gap size between the active surfaces of the rotating and stationary bearing members. The gap size is typically in the range of only 5 to 20 um, while the range of bearing diameters is typically 2 to 10 cm, its length 5 cm to 15 cm. So the gap is of relatively small size. Given a certain speed of rotation, large gaps as well as low viscosity of the bearing fluid (hot liquid metal) both cut down the loading capacity (bearing stiffness).